If all goes according to Ambri's plan, the problems with renewable energy will literally melt away.

The Cambridge, Mass.-based startup, founded in 2010, has developed liquid metal batteries that contain cheap materials that melt at 500 degrees Celsius. The cells store hours worth of energy, and unlike conventional batteries, their contents don't degrade over time.

Backed by Bill Gates and billionaire investor Vinod Khosla's venture capital firm, Ambri said Wednesday that it is expanding its team and business with $35 million in funding from those backers plus a handful of other venture firms.

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These aren't batteries that power a TV clicker or an iPhone. These store energy for entire grids, like a large campus or a military base. Because of their scale, the promise is big. The problem with wind and solar energy is that they can be inconsistent. For example, a cloud blocking the sun can cause solar panels to be effective only intermittently. Storing up energy when it's abundant with Ambri's batteries means it can be used when it's needed later.

Currently, most of "grid energy storage" -- some 99 percent, according to the Electric Power Research Institute -- comes from water-pumped systems that are expensive to operate.

"If we can get liquid-metal batteries down to $500 a kilowatt-hour, we'll change the world," Ambri co-founder Donald Sadoway, a chemistry professor at the Massachusetts Institute of Technology, told Bloomberg last month. The MIT Technology Review named Ambri one of the 50 most disruptive companies of 2013.

"It's a holy grail type problem," said Khosla Venture's Andrew Chung, a board member at Ambri, who gave Sadoway an early grant of $100,000 to help develop the technology.

The company is bankrolled in part by Gates, but that investment almost didn't happen, said Chung. When Gates, who was a fan of Sadoway's online lectures, first called him personally, Sadoway did not believe the technology icon was actually on the other end of the phone line.

"Don gave him a hard time," said Chung. "The investment almost didn't happen because Don was so protective of his time." Gates eventually invested $4 million, alongside the energy company Total, soon after the company was spun off of MIT.

Ambri is just starting to commercialize its efforts. The company has gotten funding in a handful of cities, including New York City, to build out prototype systems of its technology. New York Gov. Andrew Cuomo awarded the $250,000 grant to Ambri, along with the energy company Con Edison, in March. Last fall, the company announced the opening of a prototype cell manufacturing facility outside of Boston. Chung said the company hopes to open a full-scale factory by the end of 2015 or early 2016.